Recipro-engines and hydraulic motors, for example, have been used as mechanisms for converting linear movement into rotation. In the conventional converting mechanisms, thrust force of a piston, which linearly moves in cylinders, is converted into rotation of a crank shaft.
The conventional mechanisms, e.g., the recipro engines, the oil motors, the have following disadvantages. Firstly, the piston and the crank shaft are connected by a connecting rod. One end of the connecting rod is pivotably connected with the piston, so they mutually incline while in operation. The connecting rod and the crank are also pivotably connected by a piston pin, so they mutually incline while in operation. When the connecting rod inclines with respect to the piston and the crank shaft, 100% of the thrust force of the piston cannot be transmitted to the crank shaft. Namely, component force in the tangent direction of the crank shaft, which is divided from component force of the thrust force toward the connecting rod, is transmitted to the crank shaft, so that transmitting loss is great. Thus, the rotary torque of the crank shaft is quite lower with respect to the linear thrust force of the piston; efficiency of converting the linear thrust force into the rotary torque is low.
Secondly, the crank shaft rotates at fixed speed when the linear movement of the piston is converted into the rotation of the crank shaft, so the speed of the piston is faster near an upper dead point and slower near a lower dead point. Thus, vibration is apt to happen. This point will be explained with reference to FIGS. 20 and 21.
FIG. 20 is an explanation view showing an example of the conventional crank mechanism including the connecting rod.
In this example, stroke "S" of the piston 110 is 40 mm; length of the connecting rod 120 is 50 mm. A connecting section 125 of the crank 130 and the connecting rod 120 are located at a 0.degree. position "L", which is the furthest position from a cylinder 140, when the piston 110 is at the lower dead point P.sub.1. If the crank 130 turns 90.degree. from the 0.degree. position "L", the connecting section is at a 90.degree. position "M"; the connecting section is at a 180.degree. position "H", when the crank 130 turns 180.degree. and the piston 110 locates at the upper dead point P.sub.2.
FIG. 21 is a graph showing relationship (a solid line) between the rotational angle X.sub.c (degree) of the crank 130 and a position Y.sub.p (mm) on the stroke of the piston 110 of the example shown in FIG. 20. Note that, a one-dot-chain line shows a sine wave.
When the crank 130 is at the 0.degree. position "L", the piston 110 is at the lower dead point P.sub.1, so the position Y.sub.p (mm) of the piston 110 is 0 mm; when the crank 130 is at the 180.degree. position "H", the piston 110 is at the upper dead point P.sub.2, so the position Y.sub.p (mm) of the piston is 40 mm.
When the piston 110 moves from the lower dead point P.sub.1 to the upper dead point P.sub.2, the crank 130 turns from the 0.degree. position "L" to the 90.degree. position "M", and the piston 110 moves 15.83 mm. When the crank 130 turns from the 90.degree. position "M" to the 180.degree. position "H", the piston 110 moves 24.17 mm. Namely, the speed of the piston 110 is faster near the upper dead point P.sub.2 and slower near the lower dead point P.sub.1. So it is difficult to stably rotate the crank shaft, and a vibration is likely to occur. Near the upper dead point P.sub.2, fuel is ignited and burnt, but the speed of the piston is fast, so that it is difficult to ignite at proper timing. With this untimely ignition, the efficiency of the conventional crank mechanism is limited, and noise cannot be reduced.
To improve above described disadvantages, the inventor of the present invention has invented a rotary drive system (Japanese Patent Kokai Gazette No. 7-12199) shown in FIGS. 22 and 23. The structure of the rotary drive system will be explained.
The rotary drive system comprises: a pair of first elongated members 16a and 16b being arranged parallel in a first direction; a pair of second elongated members 18a and 18b being arranged parallel in a second direction perpendicular to the first direction; a first rod 20 being arranged parallel to the first elongated members 16a and 16b and capable of moving in the second direction in a state of being parallel to the first elongated members 16a and 16b; a second rod 22 being arranged parallel to the second elongated members 18a and 18b and capable of moving in the first direction in a state of being parallel to the second elongated members 18a and 18b; a moving body 24 being capable of moving in the first direction and the second direction along the first rod 20 and the second rod 22 in a rectangular plane 28 enclosed by the first elongated members 16a and 16b and the second elongated members 18a and 18b; a first driving mechanism 26a-d for moving the second rod 22 in the first direction; a second driving mechanism 34a-d for moving the first rod 20 in the second direction; rotary shafts 36a and 36b being capable of rotating about axial lines; and levers 38a and 38b whose one ends are pivotably connected with the moving body 24 and whose the other ends are fixed to one of the rotary shafts 36a and 36b, whereby the levers rotate the rotary shafts 36a and 36b when the moving body 24 moves round the rotary shafts 36a and 36b.
In the rotary drive system, the one ends of the levers 38a and 38b are pivotably connected with the moving body 24; the other ends thereof are fixed to one of the rotary shafts 36a and 36b, which are capable of rotating about the axial lines. So the rotary shafts 36a and 36b are rotated about their axial lines when the moving body 24 moves round the rotary shafts 36a and 36b. With this structure, the rotary drive system can convert the linear thrust force into the rotary torque without employing the conventional connecting rod, so that the converting efficiency can be improved and reducing the vibration.